The field of the invention is that of receivers for digital signals transmitted in the form of packets. More particularly, the present invention relates to a method and apparatus applied to simultaneous phase synchronization and decoding of received packets, said phase synchronization and decoding making use of the maximum likelihood criterion implemented in the Viterbi algorithm.
The invention applies in particular to receiving short packets transmitted by satellite. In such a transmission system the signal-to-noise ratio (S/N) can be very low: the bit energy to noise ratio Eb/No on a channel (or as transmitted, i.e. after encoding) is of the order of 0 or 1 dB. In conventional convolutive encoding, this represents an efficiency of xc2xd compared with the theoretically available Eb/No ratio which is of the order of 3 to 4 dB. In addition, observed frequency differences xcex94f relative to symbol time Ts (xcex94f.Ts), i.e. observed differences in frequency between the carrier of the signal as received and that of the local oscillator of the receiver relative to symbol time, are conventionally of the order of 10xe2x88x922 to 10xe2x88x923.
Such packets are usually decoded by means of a receiver system as shown in FIG. 1.
The received packets are applied to a quadrature demodulator 10 that also receives a local oscillator signal OL. The demodulator 10 supplies a baseband signal to an analog-to-digital converter 11 followed by a prefilter 12. The signal output from the prefilter 12 is applied simultaneously to a clock estimator 13 and to a filter 14 having a finite impulse response. The symbols from the filter 14 are then applied to a frequency estimator 15 which eliminates the residual frequency difference, said frequency estimator being followed by a phase estimator 16 that corrects the phase of the received signal. The phase estimator 16 may, for example, be a Viterbi and Viterbi estimator. Finally the decided bits are output by a decoder operating using the criterion of maximum likelihood, typically a Viterbi decoder 17.
The problem raised by a receiver system of that type is that the time required to acquire phase and frequency synchronization is long if phase and frequency estimating is performed on a plurality of packets (e.g. for a transmitted data rate of 25 kbauds/s and xcex94f of 600 Hz). This is particularly true when access packets are spaced apart in time and of long duration, as during call setup.
Furthermore, a system of that type operates in tracking mode, i.e. when xcex94f.Ts is about 10xe2x88x923, it has a binary error rate (BER) that is about 1.5 dB less good than the theoretical BER, which constitutes a severe penalty in satellite transmission since it makes it necessary to increase transmitter power at the satellite. This drawback is associated with the short duration of the packets, e.g. about 128 symbols, together with thermal noise and lack of phase coherence between successive packets (phase noise).
This problem, associated with the inability of frequency estimators 15 and of phase estimators 16 to provide sufficient phase correction on received symbols, has been solved by integrating the block 16 in the Viterbi decoder 17 for estimating phase and by eliminating the frequency estimator. Such integration is described with reference to FIG. 2 which shows a decision-making step as conventionally implemented in a Viterbi decoder.
In this figure, rk corresponds to a received complex decision variable, e.g. expressed on 6 bits (3 bits for each of the components P and Q of the symbol under consideration), k responding to the symbol being decoded, xcex0 and xcex1 corresponding to two calculated metrics, and bk is the bit corresponding to the decoding symbol dk.
Conventionally, the metric xcex is equal to:
xcex=∥rkxe2x88x92dk∥2
The solution enabling the phase of rk to be corrected in the Viterbi decoder consists in estimating phase over all of the previously received symbols. This estimation consists more precisely in calculating the following values:   Arg  ⁡      (                  ∑                  k          =          0                N            ⁢                        r                      k            -            n                          ·                  d                      k            -            n                    *                      )  
where Arg is the argument and N is the number of symbols participating in phase estimation.
This provides a mean value for the phase which is written xcfx86k. The following step then consists in correcting the metric xcex by calculating:
xcex=∥rkxc2x7exe2x88x92jxcfx86kxe2x88x92dk∥2
The value of xcex then takes account of phase error in the received symbol. This type of phase correction makes it possible to improve performance in terms of acquisition and tracking quite perceptibly. In addition, the modified Viterbi algorithm is very robust against phase noise and large frequency differences xcex94f.Ts.
However, the problem raised by that known solution is that in order to calculate       ∑          n      =      0        N    ⁢            r              k        -        n              ·          d              k        -        n            *      
it is necessary to calculate a sliding window. That is complex to implement. In addition, extracting the argument consumes a large amount of calculation power. Finally, that decoding method is not compatible with a coding rate other than xc2xd, e.g. with a punctured code of the xc2xe or ⅘ type (generally of efficiency l/(l+1)).
A particular object of the present invention is to remedy those drawbacks.
More precisely, an object of the invention is to provide a method and apparatus for simultaneous phase synchronization and decoding that makes use of the criterion of maximum likelihood which is easy to implement, which does not require long calculation time, and which can be compatible with any coding rate.
These objects and others that appear below are achieved by a method of simultaneous phase synchronization and decoding that makes use of the maximum likelihood criterion, the method being applied to signal packets received at a receiver, the received signals having been subjected to convolution encoding at a transmitter, the method consisting in calculating branch metrics taking account firstly of firm decisions calculated on the received symbols and secondly of a magnitude that takes account of phase error between the carrier of the received signal and the local oscillator signal used in the receiver, the magnitude weighting the decision variables constituted by the complex digital components of the received symbols, the method being characterized in that the magnitude is equal to xcexa3*k for each of the paths studied, where
xcexa3k=xcexa3Nn=( )rnxe2x88x92kd*kxe2x88x92n
r designating a received complex decision variable;
d designating the decoding symbol;
( )* designating the complex conjugate symbol of a complex value;
k designating the current decoded symbol; and
N the number of symbols taken into consideration.
Advantageously, the values of the branch metrics are:
xcexk=(xcexa3k*xc2x7rk)xc2x7dk+(xcexa3k*xc2x7rk)*xc2x7dk
In a preferred implementation, the magnitude xcexa3k is replaced by Sk where Sk is given by:
xe2x80x83Sk=xcex1xc2x7Skxe2x88x921+rkxc2x7dk*
with S0=0 and xcex1 equals a positive constant less than 1.
In another preferred implementation, in order to obtain a metric modulus that is statistically constant for decoding the first symbols of said received packets, said magnitude xcexa3k is replaced by Sk/(1xe2x88x92xcex1k), where Sk is given by:
Sk=xcex1xc2x7Skxe2x88x921+rkxc2x7dk*
with S0=0 and a equals a positive constant less than 1.
Advantageously, the received signals are subjected to puncturing at symbol level, said puncturing having an efficiency of l/(l+1), for example.
The method of the invention is advantageously applied to the phase-tracking stage in the receiver and can equally or additionally be applied to the phase acquisition stage.
Each packet preferably comprises a header comprising a single word.
The invention also provides an apparatus for simultaneous phase synchronization and decoding making use of the maximum likelihood criterion, the apparatus being designed to receive signal packets transmitted by a transmitter, the received signals having been subjected to convolution encoding at the transmitter, the apparatus comprising means for calculating branch metrics taking account firstly of firm decisions calculated on the received signals and secondly of a magnitude taking account of the phase error between the carrier of the received signal and the local oscillator signal used at the receiver, the magnitude weighting the decision variable constituted by the complex digital components of the received symbols, the apparatus being characterized in that the magnitude is equal to xcexa3*k for each of the paths studied, where:
xcexa3k=xcexa3Nn=( )rkxe2x88x92nd*kxe2x88x92n
r designating a received complex decision variable;
d designating the decoding symbol;
( )* designating the complex conjugate symbol of a complex value;
k designating the current decoded symbol; and
N the number of symbols taken into consideration.
* designating the complex conjugate, k the current decoded symbol, and N the number of symbols taken into consideration.
Advantageously, the apparatus includes means for calculating the values of the branch metrics, supplying:
xcexk=(xcexa3k*xc2x7rk)xc2x7dk*+(xcexa3k*xc2x7rk)*xc2x7dk
Optionally and advantageously, the magnitude xcexa3k is replaced by Sk, where Sk is equal to:
Sk=xcex1xc2x7Skxe2x88x921+rkxc2x7dk*
with S0=0 and xcex1 equals a positive constant less than 1, or else by Sk/(1xe2x88x92xcex1k).
The invention also provides a receiver for signal packets that have been subjected to convolutive encoding at a transmitter, the receiver including such apparatus.